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Simulators >> Circuit Simulators >> Why is cmfb stability different between STB and PSTB?
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Message started by vence on Mar 9^th, 2010, 11:56pm

Title: Why is cmfb stability different between STB and PSTB?
Post by vence on Mar 9^th, 2010, 11:56pm

I simulate sc cmfb loop stability based on two methods: stb analysis at tran=97.85u & PSTB with tstab=90u. But I get different magnitude(dB) and phase plot below 10KHz.

Why is cmfb stability different between STB and PSTB? And which one is right?

Thanks in advance.

Title: Re: Why is cmfb stability different between STB and PSTB?
Post by vence on Mar 10^th, 2010, 12:00am

And this is my testbench.

Title: Re: Why is cmfb stability different between STB and PSTB?
Post by subgold on Mar 10^th, 2010, 5:15am

i am not sure if you can simulate the circuit with stb.
althgough you might obtain the correct dc operating point from the transient simulation, the feedback factor of the cmfb loop is still uncertain, as there is no dc component in the sc-smfb block.

Title: Re: Why is cmfb stability different between STB and PSTB?
Post by sheldon on Mar 22^nd, 2010, 7:28pm

Vence,

Attached is an example of how to setup your testbench. This
testbench replicates how the circuit will be used, i.e., it includes
feedback, etc. The tricky point is that you need perform the
dc operating first with the circuit setup for "sampling": sample
the short input to reference level and short the common mode
input to the common mode reference level, and then run the
stability analysis in "gain": apply an input signal and connect
the common mode level to the feedback. This is pretty straight
forward using ADE-XL [IC61].

Best Regards,

Sheldon